1. Field of the Invention
The invention relates to a process in which both hydrocarbons and hydrogen are produced from syngas. More particularly, the invention relates to a gas conversion process for synthesizing hydrocarbons and producing hydrogen from syngas, with the hydrogen used for at least one of (i) hydrocarbon synthesis catalyst rejuvenation and (ii) hydrocarbon product upgrading.
2. Background of the Invention
Hydrocarbon synthesis processes are known in which a synthesis gas feed comprising a mixture of H2 and CO is fed into a hydrocarbon synthesis reactor in which it reacts in the presence of a Fischer-Tropsch catalyst under conditions effective to form higher molecular weight hydrocarbons. These processes include fixed bed, fluid bed and slurry hydrocarbon synthesis, all of which are well documented in various technical articles and in patents. In many cases it is desired that the synthesized hydrocarbons comprise mostly C5+ hydrocarbons (e.g., C5+-C200) and preferably C10+ hydrocarbons, at least a portion of which are solid at standard conditions of room temperature and pressure. It is preferred in a slurry hydrocarbon synthesis process that the hydrocarbons comprise mostly C5+ paraffins. These hydrocarbons are upgraded to more valuable products by one or more hydroconversion operations in which at least a portion of the molecular structure is changed by reacting with hydrogen. Hydroconversion operations therefore all require hydrogen. Hydrogen is also required for rejuvenating the hydrocarbon synthesis catalyst and sometimes for maintaining or changing the H2 to CO ratio of the syngas feed for the hydrocarbon synthesis. It is desirable to have a hydrocarbon synthesis process which generates the hydrogen required for the hydrocarbon synthesis catalyst rejuvenation and also for the hydroconversion upgrading of the synthesized hydrocarbons, rather than depending on an outside source of hydrogen.
The present invention relates to a gas conversion process for catalytically synthesizing hydrocarbons and producing hydrogen from a synthesis gas (syngas) comprising a mixture of H2 and CO, and upgrading the synthesized hydrocarbons, wherein the hydrogen is used for at least one of (a) hydrocarbon synthesis catalyst rejuvenation and (b) upgrading at least a portion of the synthesized hydrocarbons by at least one hydroconversion operation. By gas conversion process is meant to include at least hydrocarbon synthesis and hydrogen production from syngas, and also conversion of at least a portion of the synthesized hydrocarbons. By conversion is meant a process in which the molecular structure of at least a portion of the hydrocarbon in a conversion zone is changed and includes both catalytic and non-catalytic processes, with or without hydrogen as a coreactant as is explained below. In a broad sense therefore, the invention comprises synthesizing hydrocarbons and producing hydrogen from a syngas, and using the syngas produced hydrogen for at least one of the processes set forth above. More specifically, the invention comprises a gas conversion process including hydrocarbon synthesis and hydrogen production from synthesis gas comprising a mixture of H2 and CO, and conversion of at least a portion of said synthesized hydrocarbons, said process comprising contacting said synthesis gas with a hydrocarbon synthesis catalyst, reacting said H2 and CO in the presence of said synthesis catalyst and species which reversibly deactivate said catalyst, at reaction conditions effective to form hydrocarbons and reversibly deactivate said catalyst, upgrading at least a portion of said synthesized hydrocarbons by at least one conversion operation, and at least one of (a) rejuvenating said catalyst by contacting it with said hydrogen produced from said syngas and (b) upgrading at least a portion of said hydrocarbons by reacting them with said hydrogen produced from said syngas in the presence of a hydroconversion catalyst to alter their molecular structure. In further embodiments, the hydrogen produced from the syngas may be used for the hydrocarbon synthesis and/or the hydrogen production. The hydrogen is produced from the syngas using one or more of (a) physical separation means such as pressure swing adsorption (PSA), membrane separation or thermal swing adsorption (TSA), and (b) chemical means such as a water gas shift reaction. Physical means for the hydrogen production will typically be used to separate the hydrogen from the syngas, irrespective of whether or not chemical means such as a water gas shift reaction is used. In order to obtain hydrogen of the desired degree of purity (e.g., at least about 99%). While it is possible that the syngas will be obtained from an outside source, typically the syngas formation will also be a part of the gas conversion process. Thus, in an embodiment in which the syngas production is part of the gas conversion plant, the invention comprises (a) reacting a gaseous hydrocarbonaceous material, oxygen and optionally steam at conditions effective to form a syngas comprising a mixture of H2 and CO, (b) contacting a portion of said syngas with a hydrocarbon synthesis catalyst at reaction conditions effective to react said H2 and CO and form hydrocarbons and reversibly deactivate said catalyst, (c) producing hydrogen from another portion of said syngas, and (d) using the hydrogen for at least one of (i) rejuvenating said catalyst and (ii) hydroconverting at least a portion of said synthesized hydrocarbons.
The hydrocarbon synthesis is accomplished by reacting the syngas in an HCS reaction zone or reactor, in the presence of a Fischer-Tropsch catalyst, at conditions effective to form hydrocarbons and preferably C5+ hydrocarbons. As is known, during the HCS reaction, the HCS catalyst reversibly deactivates due to the presence of catalyst deactivating species, such as nitrogen compounds present in the syngas (e.g., HCN and NH3) and possibly others formed by the HCS reaction. It is also known that the catalytic activity is restored (rejuvenated) by contacting the catalyst with hydrogen or a gas comprising hydrogen. At least a portion of the synthesized hydrocarbon product removed from the HCS reactor is upgraded by at least one conversion operation, to reduce its viscosity or pour point, or to convert them into boiling fractions of higher value. Typically the conversion will comprise at least one hydroconversion operation in which the hydrocarbons react with hydrogen in the presence of a hydroconversion catalyst. It is preferred that a gas conversion plant provide at least a portion of the hydrogen needed for one or more of these uses within the plant, rather than be dependent on an outside source.
Producing hydrogen from the syngas using physical separation means provides relatively pure hydrogen, along with an offgas which comprises a hydrogen depleted and CO rich mixture of H2 and CO. This CO rich offgas may be used as fuel or fed into the HCS reaction zone. If the demand for hydrogen is greater than can be met by separating hydrogen from the syngas, or if an ancillary or alternate means for producing hydrogen is desired, chemical means such as a water gas shift reactor may be used to produce, from the syngas, all or a portion of the hydrogen required. In this embodiment, at least one of (a) a portion of the syngas and (b) the CO rich offgas resulting from physically separating hydrogen from the syngas, are fed into a water gas shift reactor in the presence of steam and a water gas shift catalyst to form a mixture of H2 and CO2 from the CO and steam, which is then passed through physical separation means to separate the H2 from the rest of the gas and form relatively pure H2, and a CO rich offgas, with the offgas recycled back into either the HCS reaction zone, into the shift reactor, or used as fuel.